טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentSomekh Oren
SubjectInformation Theoretic Aspects of Cellular Communication
Models with Fading
DepartmentDepartment of Electrical Engineering
Supervisor ? 18? Shlomo Shamai )Shitz(


Abstract

The deployment and overwhelming success of commercial cellular communication systems, based on GSM, IS-95 and recently 3G standards, spurred a worldwide intensive research effort to assess the ultimate theoretical performance limitations of cellular systems. In general, the research deals with information theoretic aspects of joint multi-cell site processing of cellular communication models in the presence of fading. Unlike conventional system models, in which each cell separately processes the signals related to its mobile users, treating other cell sites related signals as additive noise; we use a novel approach in which a finite or infinite cluster of adjacent cells are jointly processing their mobile users signals. We demonstrate, in various setups, that the joint multi-cell site processing approach yields a non-interference limited behavior and provides a significant performance enhancement over the conventional single cell-site processing approach. In order to enable an analytical framework, a class of simple system setups based on a model introduced by Wyner in 1994 is used. Although this simple model is hardly realistic, it serves as a tractable model providing considerable insight into complex and analytically intractable real-world cellular communications. Shannon-theoretic limits on the achievable throughput for the uplink channel of Wyner's model are presented. Both TDMA and Wide-Band transmission schemes are considered. For both schemes, performance enhancement as compared to single cell-site processing is demonstrated. In addition, it is observed that under certain conditions, the presence of fading increases the maximum reliable equal rate of the system. Next, chip-interleaved randomly spread DS-CDMA scheme, employed in the uplink channel of Wyner's model, is considered. Focusing on the asymptotic setup in which both the number of users per cell and the processing gain go to infinity while their ratio converges to some finite constant, the spectral efficiencies of both the optimum and linear MMSE joint multi-cell receivers are investigated. This asymptotic setup allows the use of recent results from the theory of random matrices. Performance enhancement as compared to single cell-site processing is demonstrated. Finally, the downlink channel sum-rate capacity, of a Wyner like setup, is considered. The duality between the Multiple Access Channel and the Broadcast Channel is used to derive an analytical expression for the sum-rate capacity of the system in the non-faded setup. Introducing fading, a multiuser diversity gain of the order of O(log log(K)) is observed for the sum-rate capacity when the number of users per-cell K is large. The impact of restricted cell-site cooperation is also considered.